1. Field of the Invention
This invention relates to a point-of-use water purification apparatus for decreasing or eliminating certain harmful chemicals presently found in water supplies. This invention can be used in domestic, commercial and industrial applications where pure water is required.
2. Description of the Prior Art
There is a need for improved water purity for industrial use in laboratories, pilot plants and critical processes which must meet very demanding purity specifications.
Also, there is a need for improved water purity for domestic use for many reasons, including easier washing, decreased damage to plumbing fixtures, staining and general aesthetics. Drinking and food preparation are the primary concerns addressed here. The water must be biologically safe as well as free from harmful organic and inorganic compounds and chemicals.
The main approaches to purifying water are either to separate the water from the harmful compounds, leaving these compounds trapped in a filter, or to alter these compounds so that they no longer are harmful. There are a number of commercially available systems that are directed to these approaches for improving the water purity. Among these are distillers, reverse osmosis systems, fine filters, granulated activated carbon filters, ozone generators, ultraviolet lamps and some combinations of these techniques.
One important technology used in water purification is the photoactivation of a catalyst by means of ultraviolet light. U.V. activated catalysts are especially effective since they are able to both oxidize and reduce the most stable organic compounds. They are also effective in the oxidation and reduction of many inorganic compounds.
The water to be treated is made to flow over a catalyst irradiated with U.V. light of the appropriate wavelength. Many compounds will have a tendency to be adsorbed onto the catalyst surface. While being held there, they are under the influence of the chemical species that can actively combine with them. These chemical species either are already available in the water, as for example, dissolved oxygen, or are generated by means of U.V. light acting on the catalyst. Free hydroxyl radicals, electrons, and "positive holes" are examples of the latter. These active species are able to break or rearrange the chemical bonds of the molecules adsorbed onto the catalyst surface.
Many molecules will not be adsorbed onto the catalyst surface, but these, too, will be destroyed through the influence of the catalyst. Active species created at the catalyst surface can enter into the bulk of the water and there attack the target molecules. These target molecules can have their chemical bonds rearranged so that entirely new molecules are created. Large molecules can be progressively split and rearranged so that they are "mineralized" i.e., converted to CO.sub.2, HCl and H.sub.2 O or other neutralized or harmless substances. In a like manner, inorganic substances in an ionic form may also be rendered harmless.
The physical and chemical processes involved with U.V. activated catalysts have been the subject of intense investigation over the years. Studies in the past have demonstrated that natural sunlight in combination with a catalyst such as titanium dioxide can effect significant destruction of toxic organic chemicals.
U.S. Pat. Nos. 4,268,399 and 4,304,687 teach the use of a zinc titanate catalyst to purify organically polluted water. U.S. Pat. No. 4,892,712 discloses a purifier that uses U.V. irradiation to photactivate a metal semiconductor used as a catalyst. The anatase form of titanium dioxide (TiO.sub.2) is disclosed as preferred but other metal semi-conductors such as ZnO, CdS, WO.sub.3 and SnO.sub.2 are also stated as being useful.
TiO.sub.2 catalysts have been bound to several different substrates, e.g., glass, inside of a glass tube and to small glass beads. Serpone et al. describes how TiO.sub.2 has been prepared and simultaneously embedded onto 3-4 mm glass beads by high temperature decomposition of titanium (IV) alkoxides in alcohol media, "Photocatalysis over TiO.sub.2 Supported on a Glass Substrate", SOLAR ENERGY MATERIALS, 14:121-127 (North Holland Amsterdam 1986). The use of glass beads facilitated the subsequent separation of TiO.sub.2 from the treated water. These glass beads were used many times over with no loss of TiO.sub.2 from their surface and no apparent loss of catalytic properties.
Finally, R. W. Mathews, "Solar-Electric Water Purification Using Photocatalytic Oxidation with TiO.sub.2 as a Stationary Phase", SOLAR ENERGY, 38:405-413 (1987), described the practical performance of TiO.sub.2 on a domestic water supply. He found that due to certain impurities in this water, the photoactivity of the catalyst decreased by 33% after 1700 liters had been treated. After only 20 or 30 liters, evidence of a yellow-brown discoloration began to appear on the catalyst surface.
There are many operational and economic disadvantages associated with known water purification systems. The present invention maximizes the purification of water without introducing the disadvantages of current systems.